Metal-organic frameworks are known. They are, in particular, distinguished by their porosity and can frequently be employed in applications comparable to those which are known for inorganic zeolites.
Metal-organic frameworks usually comprise an at least bidentate organic compound which is coordinated to a metal ion and joins at least two metal ions in a bridging fashion and, thus, together with the metal ions represents the skeleton of the metal-organic framework.
A suitable choice of metal and/or organic compound makes it possible to optimize the framework for the desired field of application. Here, for example, the choice of organic compound can have an influence on the pore distribution. Furthermore, the metal can make a contribution in adsorption processes.
A number of MOFs have been reported to exhibit a good adsorption/desorption behavior of economically interesting gases, e.g. methane and hydrogen.
Thus, there is an on-going task to provide porous materials exhibiting high surface area along with high heat of adsorption to pave the way into a sustainable hydrogen economy.
MOFs with magnesium as metal and phenylene based dihydroxy-dicarboxylic acids have been reported in Science 336 (2012) p. 1018-1023. The linkers differ in the number of phenylene rings that are attached to each other starting with 2,5-dihydroxyterephthalic acid IRMOF-74-I and ending with dihydroxy-dibenzoic acid groups that are separated by nine eventually substituted phenylene rings (IRMOF-74-XI). The BET surface areas for the series ranging from IRMOF-74-I to IRMOF-74-XI were found to be 1350, 2510, 2440, 2480, 2230, 1600, 1800, 1920, and 1760 m2/g, respectively.
The Mg-MOF structure exhibits a very high affinity towards hydrogen, expressed by means of a heat of adsorption of hydrogen for IR-MOF74-II [Mg2(4-(4-carboxy-3-hydroxy-phenyl)-2-hydroxy-benzoic acid)] at around 10 kJ/mol at 77K, values that are much higher compared to current best in class hydrogen storage material MOF177 (˜5 kJ/mol).
Other dihydroxy-dicarboxylic acids with connecting groups other than phenylene are rarely known. An acetylene bridged compound comprising carboxy-hydroxy-phenyl end groups is known (CAS Registry Number 1348610-42-3). However, in this compound, the carboxylic groups are in meta-position and the hydroxylic groups are in para-position relating to the acetylene group.
Symmetrical diarylacetylenes are disclosed in a generic matter in DE 39 36 297 (U.S. Pat. No. 5,185,454 and EP 425 930, respectively) and WO 91/10634.
4,4′-Ethyne-1,2-diyldibenzoate, i.e. a compound without substituents at the aromatic rings, is reported in Inorganic Chemistry 2008, 47, 6329-6335.
The synthesis of another compound without substituents at the aromatic rings, i.e. 4,4′-di-(1,4,buta-1,3-diynyl) benzoic acid and its use in two-dimensional metal-organic coordination networks is described in J. Am. Chem. Soc. 2012, 134, 6072-6075. Therein, reference is made to syntheses of eventually substituted ethynylarenes: Chem. Lett. 1998, 1099-1100; J. Mater. Chem. 2005, 15, 690-697; and J. Org. Chem. 2006, 71, 4734-4741.
A series of 4,4′-ethynylenedibenzoic acids, their preparation and use as ligands in MOFs are described in Chem. Commun. 2008, 3672-3674.